Expandable borehole plugs are generally employed to form a support or “deck” within an explosives borehole to support a column of explosive composition thereabove. Selective placement of one or more borehole plugs in a borehole enables selective concentration of explosive energy in one or more regions along the length of the borehole.
Inflatable borehole plugs usually comprise a sealed gas tight flexible bag containing a source of pressurized fluid which source, when actuated has a time delay property to enable the flexible bag to be lowered into a borehole to a predetermined depth before expanding against the borehole wall to form a deck.
Typically the source of pressurized fluid comprises an aerosol canister with a conventional valve stem and a mechanical actuator which, when actuated, holds the valve in an open position to discharge the entire contents of the canister in a manner similar to insecticide “bombs” or other aerosol fumigants.
The aerosol canister may contain a quantity of a non-expansive liquid, such as water, and a quantity of an expansive propellant, such as a hydrocarbon or halohydrocarbon, which remain in separate phases with the water at the bottom of the can and the propellant in a gas/liquid space thereabove. Alternatively, the canister may contain only a standard expandable fluid.
A dip tube or stem extends from the interior inlet port of the valve to open adjacent the base of the aerosol canister and, when actuated, water, if present, is discharged first and when substantially all of the water volume is discharged, the propellant gas is then discharged. In this manner, the water acts as a time delay fluid enabling the inflatable borehole plug to be lowered to a predetermined depth in a borehole before the propellant gas enters the flexible bag body of the plug to expand the body against the borehole wall.
The period of time delay can be as much as 5 minutes and is influenced by the volume of water in the aerosol can and/or the diameter of the discharge orifice in the valve and/or actuating cap.
Expandable borehole plugs or modified forms thereof are described in Australian Patent 763474 and Australian Patent 779463, the disclosures of which are incorporated herein by reference.
There are limitations to the control over inflation delay which can be exerted purely by means of the inflation fluid composition. Further, in many cases the time delay effected by the inflation fluid is greatly in excess of the time required to place the borehole plug at the desired depth in the borehole. This excess time is inefficient and a waste of skilled labour hours. It would be valuable to provide for a further control mechanism over the inflation process and, particularly, allow the process to be sped up, when required, without requiring a range of inflation fluid compositions be provided, tailor made for different depth applications.